Many types of thermal printing papers or "thermally sensitive record materials" have been known in the prior art. Such thermally sensitive record materials typically comprise a base sheet of paper or similar material having a film-like heat sensitive record layer disposed thereon. The heat sensitive record layer generally contains dispersions of chemical materials which, upon application of heat and/or pressure to the record layer, will undergo a darkening or coloring reaction to result in the formation of a colored or darkened image on the record material.
The particular reactive chemicals contained in the heat sensitive record layer will generally dictate the type, density and color of the image formed therewithin.
Many heat sensitive record materials, including the majority of those used in commercial labeling applications, have record layers which contain a first reactive chemical known as a "color former", or color developer and a second reactive chemical known as a "dye precursor". The color former and dye precursor are capable of reacting with one another to form a desired colored image on the record material. The color former and dye precursor chemicals may be separately microencapsulated or otherwise separately contained so as to prevent casual mixing with one another as would likely result in the formation of a visible image on the record material. However, when the microencapsulation or other containment means is disrupted by the application of heat or pressure to the record layer, the dye precursor and color former chemicals contained therein will be permitted to flow together and react, thereby forming the desired visible image on the record layer. It is by such mechanism that thermal printing heads are utilized to form printed images on heat sensitive record materials.
Numerous types of dye precursors and color formers have been utilized in the heat sensitive record materials of the prior art. Prior to the image forming reaction, the color former(s) and dye precursor(s) remain inherently colorless (e.g. pale or white in color), when the image forming reaction takes place between the dye precursor(s), and becomes darkened in color, thereby forming the desired visual image. Accordingly, a group of light colored alkaline dye compounds known as "leuco" dyes are frequently employed as dye precursor components in the record layers of heat sensitive record materials. Examples of leuco compounds which may be usable for this purpose include: auramine leuco compounds, spiropyran leuco compounds, phenothiazine leuco compounds, fluoran leuco compounds, and triphenylmethane leuco compounds. In particular, the fluoran leuco compounds have the following general structural formula: ##STR1## Where in R.sub.1 and R.sub.2 each represent an alkyl or cyclohexyl group, R.sub.3 represents an alkyl group, haloalkyl group or a halogen atom and R.sub.4 represents either hydrogen or an alkyl group.
The following compounds are examples of fluoran leuco dye precursors:
3-diethylamino-7-o-chloro-anilinofluoran; PA0 3-diethylamino-7-m-chloro-anilinofluoran; PA0 3-di-n-butylamino-7-o-chloro-anilino-fluoran; PA0 2-(N-3'-trifluoromethylphenyl)amino-6-diethylamino-fluoran; PA0 2-(N-3'trifluoromethylphenyl-N-methyl)amino-6-diethylaminofluoran; and PA0 3-diethylamino-7-(3'trifluoromethylphenyl)amino-4'-chlorofluoran. PA0 N,N'-diphenylthiourea; PA0 4,4'-diethylphenylthiourea; PA0 4,4'-diechlorophenylthiourea; PA0 3,3'-dichlorophenylthiourea; PA0 3,3'-dimethylphenylthiourea; and PA0 3,3'ditrichloro-methylpehnylthiourea. PA0 Tinuvin 1130 benzotriazole UV absorber (Ciba-Geigy, Hawthorne, N.J.); PA0 4-methylbenzophenone; PA0 2,4,6 -trimethylbenzophenone; PA0 2-hydroxy-2-methyl-1-phenylpropanone; PA0 oligo [hydroxy-2-methyl-1-4(1-methylvinyl)phenyl]propanone; PA0 Benzidimethyketal; PA0 Thioxanthones [e.g. 2-isopropylthioxanthone]; PA0 Acethphenones [e.g. 2,2-diethoxyactophenone]; PA0 Amino Benzoates [e.g. ethyl 4-(di-methylamino)benzoate]; PA0 2,4,6 trimethylbenzoylddiphenoylphophine oxide; and PA0 Aromatic Ketones [e.g. 1-hydroxycyclohexyl phenyl ketone].
The color developers which combine with the leuco dye precursors to form the desired visual image include such compounds as phenolic compounds, sulfur compounds, esters, carboxylic acids, metal salts and amines. In particular, phenylthiourea color developer compounds of the following general structural formula are usable in connection with the various leuco dye precursors: ##STR2## Where x and y each represent a halogen atom, alkyl group or a haloalkyl group, and m and n each represent an integer of 0 to 3.
The following compounds are specific examples of phenylthiourea color formers which may be used in connection with the various leuco dye precursors.
For purposes of illustration, a typical image forming reaction between a leuco dye precursor and a color former is shown herebelow: ##STR3##
In addition to the image forming chemicals (e.g. the dye precursor and color former) the thermosensitive record layer may contain one or more binder agents, fillers, surfactants, and/or lubricants to minimize wear on the thermal head as the thermal head travels over the thermosensitive record layer.
One problem associated with thermosensitive record materials of the prior art is poor resistance to ultraviolet radiation. This poor resistance to ultraviolet radiation is manifested in two ways. First, the thermosensitive record layer tends to darken or yellow upon exposure to ultraviolet radiation. Second, thermally printed images formed on the thermosensitive record material tend to undergo fading or "photodegradation" when exposed to ultraviolet radiation. Both of these adverse effects of ultraviolet radiation represent functional drawbacks in the routine commercial usage of thermosensitive record materials in many commercial labeling and packaging applications.
Notably, product labels and inventory documents having optically readable bar codes formed thereon have become widely used in many commercial applications. However, the use of thermally sensitive record materials in the printing of such labels and documents has been inhibited due to the fact that darkening of the label and/or photodegradation of the bar code may sometimes result in diminished contrast between the bar code relative to the label background, thereby interfering with the routine optical scanning and reading of such bar codes.
Also, ultraviolet induced fading or photodegradation of printed images has been known to be exaggerated in printed articles wherein the image is formed of fluorescent dye or ink, as such fluorescent dyes or inks are known to be particularly susceptible to photodegradable or fading upon exposure to ultraviolet radiation.
In view of the above-described problems associated with the use of thermally sensitive record materials or ultraviolet sensitive materials in ultraviolet exposed applications, prior attempts have been made to provide ultraviolet protective overcoats which may be applied to paper, cardboard, label stock or thermally sensitive record material for the purpose of absorbing or preventing the transmission of ultraviolet radiation into the thermosensitive record layer.
Examples of ultraviolet protective overcoats for thermally sensitive record materials are found in U.S. Pat. Nos. 4,886,774 (Doi), 4,663,642 (Kameda, et al.) and 4,942,204 (Yamamoto, et al.).
Although the prior art has included some ultraviolet protective overcoats capable of protecting printed matter from the adverse effects of ultraviolet radiation, the application and curing of such ultraviolet protective overcoats of the prior art may be less than suitable for all types of printed matter and/or record material substitutes, due to the chemical content and/or processing requirements of such ultraviolet protective overcoats.
Accordingly, there remains a need in the art for improved ultraviolet protective overcoats capable of being applied to printing substrates such as paper, cardboard, plastic film, thermally sensitive record material, and the like, to prevent ultraviolet radiation from causing yellowing or darkening of the substrate and/or fading or degradation of words or images printed on the substrate without substantially increasing the time, expense or complexity of the manufacturing process.